This invention relates to an automatic speed control system for a vehicle, such as a heavy duty truck, that is provided with a source of air pressure.
"Cruise control" has become an increasingly popular option on passenger cars in recent years. The cruise control, or speed control, option permits the vehicle operator to maintain a predetermined speed while being able to remove his foot from the accelerator pedal. The cruise control option has obvious advantages in reducing driver fatigue, and also provides a marginal increase in fuel economy, since a steady speed is maintained and the vehicle is not repeatedly accelerated and decelerated. Since heavy duty trucks are commonly used on long distance routes, the inherent advantage of cruise or speed control for automobiles is also important on long-haul trucks. However, until now, trucks have not been equipped with speed or cruise control devices.
One problem in providing a truck with a speed control system is the fact that, as compared to passenger cars, trucks are relatively underpowered, and do not have the capability to accelerate quickly in order to maintain the speed set by the speed control. For this reason, a truck speed control must be more sophisticated than an automotive speed control, and must provide proportional control of the vehicle throttle, instead of the "on-off" control commonly provided by passenger car vehicle cruise controls.
Another factor making truck speed control desirable is the fact that trucks are often equipped with accessory devices that require a predetermined constant vehicle engine speed for most efficient operation. An inherent advantage of a truck cruise control system is the fact that it can also be made to regulate engine speed, thereby providing an engine speed control for use when the truck engine is used to operate auxiliary devices.